Continuously variable transmissions of the class broadly characterizable as that in which a belt couples a pair of pulleys, each of which can assume a more or less continuous range of effective diameters, generally fall into two categories; viz.: (a) those employing V-belts or variations thereof (such as link belts or chains) for transmitting power from one pulley to the other and (b) those systems employing flat, flexible belts between the variable diameter pulleys.
Those skilled in the art have come to appreciate that CVT's employing flat, flexible belts enjoy significant fundamental advantages over those systems employing V-belts. In the case of the latter, the belts are composed of various compositions and have a trapezoidal cross section, the belt transmitting rotary motion at one speed from a source of power (such as an engine or motor) to an output shaft at another speed, the speed ratio being varied in a continuous fashion from a minimum to a maximum as dependent on the geometry of the belt and the pulley system. The V-belt is compressed between smooth, conical sheave sections in each of the two pulleys by external axial forces acting on the sections to apply tension or compression to the belt and friction between the sides of the belt in the sheave sections to prevent slippage. In operation, a force unbalance caused by changes in the axial loading of the sheave sections causes the V-belt to change its radial positions in the two pulleys until a force balance is achieved or a limit range stop is reached.
For a large transmitted torque, the required axial forces exerted on the sheaves result in large compressive forces on the V-belt which requires that the belt have a substantial thickness to prevent its axial collapse or failure. This increase in thickness increases the belt's centrifugal force and causes higher belt tension load. In addition, as the belt thickness increases, the pulley size must be increased due to higher stress loads at a given design minimum pulley radius. Further, the typical V-belt must continuously "pull out" from the compressive sheave load on leaving each pulley which results in significant friction losses and belt fatigue which adversely affects the overall efficiency of the system and the operating life of the belt. Consequently, although variable speed pulley drives have successfully used V-belts in a wide range of applications, they have been severely limited in their power capabilities for more competitive smaller size equipment.
As a result of these inherent drawbacks to the use of V-belts in continuously variable transmissions, a second category has developed which may broadly be designated as flat belt drive continuously variable transmissions. As the name suggests, flat belts are employed between driven and driving pulley assemblies which are dynamically individually variable in diameter to obtain the sought-after ratio changes. No axial movement between the two pulley rims is necessary. On the other hand, it is necessary to somehow effect the diametric variations of the individual pulley assemblies, and in one particularly effective system, this function is achieved by causing a circular array of drive elements in each pulley to translate radially inwardly or outwardly in concert as may be appropriate to obtain a given effective diameter of the pulley assembly. Variable speed flat belt transmissions of this particular type, and their associated control systems, are disclosed in U.S. Pat. Nos. 4,024,772; 4,295,836; 4,591,351 and 4,714,452 as well as U.S. patent application Ser. No. 051,922, filed May 19, 1987, and now U.S. Pat. No. 4,768,996 and Ser. No. 132,783, filed Dec. 14, 1987, all issued to Emerson L. Kumm. In all but the first patent enumerated above, the subject variable diameter pulley components have included a pair of pulley sheaves between which there extends a series of belt engaging elements that are simultaneously moved both radially and circumferentially. In one exemplary construction, there is a series of twenty-four belt engaging elements such that an angle of fifteen degrees extends between runs of the belt coming off tangentially from one belt engaging element compared to that of an immediately adjacent belt engaging element.
Each pulley assembly includes two sets of two disks (designated, respectively, the inner guideway disk and the outer guideway disk in each pair) which are parallel to each other with the inner and outer guideway disks of each set being disposed immediately adjacent one another. Each of the guideway disks of an adjacent pair has a series of spiral grooves or guideways with the guideways of the pair oriented in the opposite sense such that the ends of the belt engaging elements are captured at the intersections of the spiral guideways. Thus, radial adjustment of the belt engaging elements may be achieved by bringing about transient relative rotation between the inner and outer guideway disks to change their angular relationship, this operation being, of course, carried out simultaneously and in coordination at both sets of guideway disks of a pulley assembly.
It is well known in the art that a pulley can transmit more torque with reduced belt slippage if an idler is used to increase the belt length contacting the pulley circumference. Such an arrangement brings about an increase in the belt wrap angle which results in increased area of contact between the belt and the pulley surface. However, idlers for this purpose have been positioned such that the belt wrap angle actually decreases on a given pulley as the effective diameter of that pulley decreases. Further, in the flat belt CVT environment, the outside dimensions of the driving and driven pulleys are typically closely spaced to achieve a compact structure. This invention achieves a practical approach which permits a dynamically variably positioned torque increasing idler to be used within the variable geometry and space restrictions of a flat belt CVT.